This invention relates to a CRT (cathode-ray tube) having a novel internal coating for suppressing arcing therein during the operation thereof; and particularly for suppressing flashovers in a CRT having a glass neck with an electron-gun mount assembly housed therein.
A color television picture tube is a CRT which comprises an evacuated envelope including a viewing window that carries a luminescent viewing screen, and a glass neck that houses an electron-gun mount assembly. During operation of the tube, the mount assembly produces one or more electron beams for selectively scanning the viewing screen so as to produce a viewable video image thereon.
During the operation of the tube, an excessive amount of stray or uncontrolled electron emission and electrical leakage sometimes develops within and/or around the structure of the mount assembly. This condition may result in flashovers, a form of arcing, which may degrade the performance of the tube and/or may be destructive of the tube and/or associated circuitry. Excessive stray or uncontrolled electron emission, leakage and arcing are a result of a combination of factors involving the electrodes and the closely-spaced glass surfaces of the neck. Various structures on the internal surfaces of the neck or on the mount assembly for suppressing such arcing have been suggested.
U.S. Pat. No. 3,355,617 to J. W. Schwartz et al. discloses an internal, electrically-resistive coating of iron oxide on the neck. U.S. Pat. No. 3,758,802 to T. Kubo et al. discloses an internal coating of crystallized glass on the neck. U.S. Pat. No. 3,979,632 to E. A. Gunning et al. discloses an internal coating of insulative chromic and/or ferric oxide on the neck. U.S. Pat. No. 4,285,990 to K. G. Hernqvist discloses an internal coating of chromium metal on the neck. Each of these prior coatings is located on the inside surface of the glass neck opposite the electrodes which produce focusing fields for the electron beam. While each of these coatings may have had a beneficial effect in some tube designs, the beneficial effect has not been great enough to find much commercial use. All of these coatings are of inorganic materials which, it is believed, permit alkali ions in the underlying neck glass to move to the internal surface thereof, producing an electronically-active surface from which "blue glow" and arcing can be supported when the tube is operated. Also, when the tube is electrically processed by spot knocking, undesirable particles of the coating are usually released into the tube and interfere with its proper performance.